Dew point hygrometer



i Klil'tKl-.Nljt 2,466,696 l FIPBZIZ XR A F. A. FRISWOLD EI'AL DEW POINTHYGROMETER Filed Sept. 25. '1945 pril l2, 1949.

tMwuNln *SHQ w Se 0 5 s as a g ,1x

vwc/rvlo/b', v

Frank A. Frl'swo/d Ralph D. Lew/'s .asigasiaiia-19e Frank A. Friswold,Cleveland, Lew-is, Parma, Ohio and Ralph D.

Application September 25,1945. Serial No. 018,804

V Claims. (Cl. 236-44) (Granted under the act of March 3, 1883. al

amended April 30, 1928: 370 0. G.. 757) This invention relates tohysrometer apparaltus and in :particular to such apparatus of thedew-point type.

'I'he general object of the invention is to provide a-hygrometer of thetype referred to which is more stable and reliable in operation.

Another object is to provide a dew-point hygrometer in-which thermal lagis reduced to a negligible factor. thereby improving its response'-characteristic to rapidly changing variations in humidity as is found inaeroplanes flying at high;

speeds through clouds, etc.

Another object is to provide a mirror type. dewpoint hygrometerutilizing a novel photronic relay in which the average heater currentfor the mirror is not affected by changes in intensity of the ligh-tsource associated `with the photo-cell elements or by changes inoperating voltages of the power supply source.

These and other objects of the invention will become more apparent fromthe detailed description to follow when considered .with theaccompanying drawings which :illustrate a preferred embodiment of theinvention.. c

Inthe drawings,

Fig. l ls a schematic wiring diagram of theeiectrical circuits of ourimproved hygrometer: and

Fig. 2 is a longitudinal section showing the retion prior to ourimprovements was that the mirror temperature was subject to aconsiderable amount of hunting. The source of heat was a resistorimbedded in the mirror head through which the heating current flowed.Because of the thermal lag of the mirror head and the large quantitiesof heat involved, a serious hunting eflect took place when the dew-pointchanged. It was not unusual to observe a five minute lag period duringwhich equilibrium conditions were being re-established. This huntingeffect made the determination of the dew-point difficult, especiallyunder operating conditions where it is sought to record rapidly changinghumidity conditions such as is experienced in aircraft ilight.v

From the description of our improved hygrometer construction which nowfollows, it will become apparent 'that the-above referredtodisadlatlonship between the various operating components of theapparatus and illustrating the circuit components in block form. 2

Dew-point type hygrometers using a cold mir- 4ror to which heat isapplied until the moisture thereon is evaporated and controlling theheater current for the mirror as a function of the amount of light froma light source which falls upon a photo-cell unit after reflection fromthe mirror are not broadly new. However, one disadvantage of this typeof hygrometer prior to our improvement was that variations in intensityof the light 4 source caused considerable changes in the temperature ofthe mirror thus introducing an error `into the hygrometer. sincehumidity is determined as a function of the temperature of :the mirror.Inasmuch as illumination varies as the eighth power of the filamentcurrent in an electric lam-p. it is obvious that lack of proper currentregulation can introduce serious errors. Aging effects in the lamp alsoare contributing factors.

Another disadvantage of hygrometer construc- .vantages are all overcome.Referring now to the drawings, there .is included a box lli of Bakelitedivided into three-sections; a coolant compartment or heat sink il. amirror compartment l2,v

and aphoto-cell compartment I3.-

' To the Bakelite `walls I0 of coolant compartment ll are secured innerwalls I4 of insulation material suchas 'a balsa which are lined withbronze sheeting l5. The coolant compartment Il is'covered with aclose-fitting lid also of Bakelite and balsalined with sheet bronze.This lid isv not shown because the detalls of'construction of box l0donot constitute the invention'claimed inthis application. A copper rodI 8 of '1/2" diameter runs thelength of the compartment Il near thebottom and is soldered at each end to the bronze sheeting I5. A smallercopper rod il of ya" diameter is set in a hole provided in one end ofrod I8 and extends throughthe balsa wall. I4A to the mirror compartment42.? When the hygrometer is in use. compartment H is filled with CO: Ilor any other suitable 4type-of refrigerant. Rods il, Il and .the CO:'ll'thus constitute a heat sink or heat absorber for pulling down, byconduction. the temperature o f the mirror element. The latter isconstltutedby a nickel cap 2l of V4" diameter fitted on theend of thesmall rod i1, and which is polished on one face 22 to act as the mirror.Nickel is preferred as the material for the cap. 2l in view of itsfavorable resistivity, thermal conductivity, magnetic characteristics.and its ability to take a high polish.

the side just beneath the surface 22. Into this hole there is inserted a82 gage iron-constanten thermocouple 28. Thermocouple 28 is connectedvia conductors 24, 28 to a temperature measuring device 28 ofconventional construction to -thereby measure the temperature of themirrored face 22 of the cap 2|.

The seams ci Bakelite walls I8 defining the mirror compartmen-t I2 aresealed with Permatex to make the compartment leak-proof. A Lucite window(not shown) may be located near the top of this compartment to permitobservation of .the mirrored cap member 2|. When the instrument is inuse. this window is covered with a Bakelite lid to keep out stray light.Brass tubing connections 21, 28, which extend through opposite walls ofcompartment I2 .to the interior thereof, serve as inlet and outlet,respectively, for the air under humidity test.

The photo-cell compartment I8 contains a type 920 photocell 8| which iscomprised of two photosensitive cell umts 8|a and vllb. Also included inthis compartment is a 32candle power lamp 38 and a lens system $4supported within a tubular housing 28 that is secured to a wall 88 oftransparent plastic such as Lucite for separating compartments I2 fromIl. Light from lamp 88 is focused by lens 84 onto the mirrored face 22of cap 2| and is reflected back through wall 88 towards the photo-cell8Ib. A cover member 81 is provided over the twin photo-cell unit 8| sothat all direct light from the lamp 88 as distinguished from thatreflected from the mirrored face 22, has no effect on the photo-cellunit 8|. However, cover 31 is provided with a slidably mounted door 38cooperative with an opening in the cover to permit a small amount oflight to fall on photocell 3|a. This arrangement is desirable to permitbetter balancing of the bridge circuit in which the photo-cells 3-|a and8|b are connected. The Lucite wall 36 is covered with an opaque coating39 except where it is necessary for the direct and reflected light raysto pass through.

The electronic components of our improved hygrometer are shown in thecircuit diagram of Fig. l. In this diagram, it is seen that photo-cellsSla and 3 Ib are connected in a Wheatstone bridge which includes abattery 48 which functions as a power source for the bridge, anotherbattery 4| which supplies a grid biasing potential for a pair ofamplifier tubes 42, 42, resistor 44 and resistor 45.- One arm of thebridge network consists of photo-cell 3|b and resistor 48, and the otherbranch of the bridge network is constituted by photo-cell -3I a andresistor 44. Initially, the photo-cell Sla and 3|b are so arranged thatwith no condensation of air on the mirrored face 22 of cap 2|, the beamof light from the lamp 88 when reflected by the mirrored face 22 willpass by the edge of photo-cell 8|b along path a. Contact arm 46 is thenadjusted until the voltage drop across resistance 44 is equal to thedrop across the resistance 45. Door 38 is adjustable to balance theamount of strap light falling on the photo-cells 3|a and 8|b. Underthese conditions, no voltage will appear across the neutral points 41,48 of the bridge network. and the latter is therefore in balance. Withthe bridge now in a balanced state, variation in intensity of the lightfrom source 33 will have no effect upon the accuracy of operation of theapparatus.

A conductor leads from point 41 to the control grid 42a of tube 42, andconductor 82 leads from point 48 on the bridge to control grid 43a oftube 43. The cathodes'42b and 48h of the tubes A- s l 42 and 48,respectively, are tied together and connected to the negative terminalof battery 48. It is thus seen that resistor 44 is connected in thecathode-grid circuit of tube 42, and the resistor 48 is similarlyconnected in the cathodegrid circuit of tube 48.

The anodes 42e and 48o of tubes 42, 48 are connected to opposite ends ofa load resistor I8. This latter resistor is connected in series with avariable grid biasing potentiometer 84 to the cathode-grid cilrcuit oftube 88, the cathode being indicated by 88a and the grid by 88h. Theanode 88e of tube 88 is connected to a plate resistor 88, and thecircuit elements are such that maximum current flows in theanode-cathode circuit of tube 88, and hence also through resistor 88.when no light from the lamp 88 as reected by the mirrored surface 22falls on the photo-cell 8|b. The voltage drop across resistor 88 is thenat a maximum. The drop across the plate resistor 88 is applied throughresistor 81 to the screen grid 88a of an oscillator tube 88 which is a6L6 operated at a frequency of two megacycles. The circuit for theoscillator 88 is constituted by a tuned grid component which includesinductance 88 and variable condenser 88, and a tuned plate circuit whichcomprises a plate inductance 8| and variable condenser 82. Resonance ofthe grid plate circuit of the oscillator 88 is indicated by a flashlightbulb 88 which is energized by a single loop 84 around plate inductance8|. A coil 88 for heating the mirrored surface 22 of cap 2| by inductionis placed over the cap 2| as shown. The high frequency magnetic fieldproduced in the coil threads through the cap 2| causing the flow of eddycurrents therein. Since these currents tend to flow on the surface,substantially only the mirrored surface portion 22 of the cap 2| isheated. Connected in circuit with coil 85 is a radio frequency ammeter88, a variable condenser 81, and an inductance 88 which is coupled tothe plate inductance 8|. The series connected condenser 81 is providedfor tuning the circuit of the heating coil 88 to resonance whichcondition is indicated by maximum deflection of the ammeter 88.

The frequency of the heating current should be so chosen that theeffective depth of penetration of the eddy currents satisfy a conditionof minimum heat dissipation with accurate thermocouple indication. For acap member 2| of the size described, a frequency of two megacyclesproved quite satisfactory.

Operation The bridge network containing photo-cells 8|a and 8|b, anddoor 88 are adjusted until the milliammeter 1| connected across theneutral points 41, 48 of the bridge reads zero. Under this condition,the grid-cathode circuits of the amplifiers 42 and 48 have the samebias, and hence there will be no difference of potential between the endterminals of resistor 83.

The two-megacycle oscillator 88 is next tuned to resonance. Thiscondition will be reached when lamp 82 reaches its maximum brilliance.

The load circuit controlling heater coil 88 is then tuned until ammeter88 reaches a maximum.

After the above adjustments have been made, the solid C0: or any othersuitable type refrigerant is placed in the coolant chamber Il, and theequipment is now ready for operation for ascertaining the humidity ofthe air passed through chamber I2.

The temperature of the cap 2| is now seen to be controlled by twofactors. One is the heating eifect from the coil Il and the other, thecooling eii'ect of the C0: i2, taking heat away from cap 2| byconduction throughrods Il and I1. It was previously explained that withno condensation on the mirrored face 22, the beam of light passes by theedge of photo-cell 2lb along path a. However, when condensation on face22 begins to take place, the light beam from lamp 22 is dispersed uponreflection from the mirrored face 22, and some of the light now passesalong path b into the photo-cell 2lb. The bridge circuit includingphoto-cells Ila, 2lb and resistors M, 6l now becomes unbalanced, and avoltage appears across bridge terminals 41, 4I. 'I'his voltage is thenimpressed across the grids 42o-42a of tubes 42 and 42, respectively, andchanges the drop across load resistor 52 driving grid Bib more negativewith respect to cathode 55a. This, in turn.

decreases the current flow in the anode-cathode circuit of tube Bl anddecreases correspondingly the voltage drop across resistor I8. Thisincreases the voltage on the screen grid lla of oscillator tube 58,thereby increasing the amplitude of the current or oscillation appearingacross the tuned output circuit comprising capacitor 62 and coil 6I.This. in turn, causes an increase in the high frequency (2 megacycles)alternating current flowing through coil Il, and increases the heatingof the mirrored surface 22 until the condensation thereon begins toevaporate. As evaporation progresses, the dispersion of light on themirrored surface 22 also decreases. Less and less light now falls on'photo-cell 2lb, and the bridge circuit is brought more and more into abalanced state. A reverse efi'ect is then Produced on resistors 53 and58. thus decreasing the voltage on screen grid 58a of oscillator il andhence also the current ow in coil BI and through heating coil 65. Whenthe temperature' of the surface 22 of a cap 2i thereby is so meedeed ireduced that condensation begins to form again,

light once again begins to fall on photo-cell 2lb, and the cycle isrepeated.

This process of alternate heating and cooling of the surface portion 22is substantially continuous. It is evident that the result is thatmirrored surface 22 is maintained at a point of equilibrium wheretheamount of heat induced in cap 2i from coil 65 substantially balancesthe amount of heat taken away from cap 2i by conduction through rods ISand I1 so that the temperature of mirrored surface 22 is maintainedsubstantially at the dew-point of the atmosphere in chamber l2. As themoisture content of this atmosphere changes, the temperature of thecondensation will likewise vary andcause the equilibrium point to shiftand the temperature of the mirrored surface 22 to vary accordingly. Aspreviously explained, thermocouple 23 imbedded in cap 2| immediatelybeneath the mirrored surface 22 measures the temperature thereof and maybe recorded by conventional recording apparatus 2l.

The invention may be embodied in other specie forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects vasillustrative and not restrictive. the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental 8purposes without the payment of any royalties thereon on therefor.

We claim:

1. A dew-point type hygrometer comprising. a mirror, a heat sinktherefor, a heat conducting element extending from said mirror into saidsink, a light source, a photo-cell, said light source being arranged todirect light towards said photo-cell by reflection from said mirror andin auch manner that the amount of light falling on said photo-cell whensaid mirror is wet differs from that falling on said photo-cell whensaid mirror is dry, a coil extending around said mirror only, means toenergize said coil with high frequency current-to thereby heat thesurface of said mirror by induction, and circuit means including saidphoto-cell for controlling operation of said mirror heating means.

2. A dew-point type hygrometer comprising. a mirror, a heat sinktherefor, a heat conducting element extending from said mirror into saidsink, a light source, a photo-cell, said light source being arranged todirect light towards said photocell by reflection from said mirror andin such manner that more light from said light source strikes thephoto-cell when said mirror is wet than when it is dry, a coil'surrounding only said mirror in proximity to the surface thereof, meansto energize said coil with high frequency current to thereby heat saidmirror by induction, and circuit means including said photo-cell forcontrolling operation of said mirror heating means.

3. A dew-point type hygrometer comprising. a mirror, a heat sinktherefor, a light source, a

first photo-cell, said light source being arranged to direct lighttowards said first photo-cell by reflection from said mirror and in suchmanner that the amount of light falling on said photocell when saidmirror is wet differs from that falling on said photo-cell when saidmirror is dry, a coil associated with said mirror, means to energizesaid coil with high frequency current to thereby heat said mirror byinduction. a second photo-cell, means connecting said first and secondphoto-cells in a bridge, and means connecting the output of said bridgeto control operation of said mirror heating means.

4. A dew-point hygrometer comprising, a mirror. a heat sink therefor, alight source, a first photo-cell, said light source being arranged todirect light towards said photo-cell by reilection from said mirror andin such manner that the amount of light falling on said photo-cell whensaid mirror is wet diners from that when said mirror is dry, a coilassociated with said mirror, means to energize said coil with high fre--quency current to thereby heat said mirror by induction, a secondphoto-cell, a bridge circuit, said bridge circuit including saidphoto-cells in opposite branches thereof, and means connecting theoutput of said bridge circuit to control operation of said mirrorheating means.

5. A dew-point type hygrometer comprising, a mirror, a heat sinktherefor, a light source, a rst photo-cell, said light source beingarranged to direct light towards said photo-cell by reflection from saidmirror and in -such manner that the amount of light falllng'on saidphoto-cell' when said mirror is wet differs from that when said mirroris dry, a coil associated with said mirror. means to energize said coilwith high frequency current to thereby heat said mirror by induction, asecond photo-cell, a bridge circuit, said bridge circuit including saidphoto-cells in different branches thereof, means connecting the outputof said bridge circuit to control operation Quo 7 8 0f md mi' was mm. mdm88!!! dim' REFERENCES CITED in: a. relatively small amount of lightfrom said source into said second photo-cell to thereby The followingreferences are ot record in the compensate for stray light falling onsaid photonle oi this patent:

n fm 1d im u- 5 UNrrnD s'rA'rm PATENTS I mm A. nuswom. Number Nm v Dm.

ma D, Llwxs, 2,059,976 Btsrzardter Nav. 3, 1938 2,268,785 A'rxmmzmmmeJan. a, 1942 n .k 5 I' J

